Mass flow meter systems and methods

ABSTRACT

A flow meter system that calculates mass flow rate based only on a single pressure signal. A flow controller is arranged in parallel with a restriction such that a constant pressure differential is maintained across the restriction. The pressure, and temperature if not controlled, of the fluid flowing through the restriction is measured on either side of the restriction. The pressure is compared to a plot of pressure versus mass flow rate calculated for the specific restriction and fluid being measured. The constant pressure differential maintained across the restriction yields a linear relationship between pressure and flow rate. If temperature is not controlled, the plot of pressure versus mass flow rate will remain linear, but the slope of the curve will be adjusted based on the temperature of the fluid.

[0001] This patent application claims priority based on provisionalpatent application U.S. Serial No. 60/283,596 filed on Apr. 13, 2001which is fully incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to systems and methods formeasuring and controlling mass flow and, more specifically, to suchsystems and methods that allow precise measurement of mass flow using aflow restriction and pressure and temperature sensors.

BACKGROUND OF THE INVENTION

[0003] In many disciplines, the mass flow of a fluid must be measuredwith a high degree of accuracy. For example, in medical andsemi-conductor manufacturing, gasses and liquids often need to bedelivered in precise quantities to obtain desired results. Meters areused to measure the mass of the fluid actually delivered.

[0004] Conventional pressure-based mass flow meters employ a flowrestriction, a temperature sensor, and pressure sensors for detectingthe absolute pressure upstream of the flow restriction as well as thedifferential pressure across the flow restriction. Mass flow isdetermined from a table that correlates the pressure and temperaturereadings with predetermined mass flow rates. Such systems require atleast two pressure sensors and a temperature sensor to account for fluiddensity, fluid velocity, and fluid viscosity under differenttemperatures and upstream and downstream pressures.

[0005] The need exists for mass flow meters that are simpler and requireless complex calculations to determine true mass flow.

RELATED ART

[0006] U.S. Pat. No. 5,791,369 to Nishino et al. discloses a flow ratecontroller that, purportedly, requires only one functional pressuretransducer. However, the controller disclosed in the '369 patentoperates only in the sonic flow regime, and this system requires thatthe inlet pressure be twice the outlet pressure for the controller tofunction properly. The flow controller of the '369 patent thus operatesonly with very low flow rates, only with gases, and must have effectivepressure regulation upstream. In addition, the '369 patent discloses theuse of a second pressure transducer to determine when the downstreampressure is more than half of the inlet pressure, and the controllershuts down when this condition is met.

[0007] U.S. Pat. No. 6,152,162 to Balazy et al. discloses a fluid flowcontroller that requires two pressure measurements, one upstream and onedownstream of a flow restrictor. The '162 patent does not measure massflow. The '162 patent also employs a filter element as the flowrestriction. Particles in the gas stream can clog the filter, therebychanging the relationship of pressure drop and flow characteristics oftheir flow restriction and possibly deviating from the initialcalibration setting.

[0008] U.S. Pat. No. 6,138,708 to Waldbusser discloses a pressurecompensated mass flow controller. The system described in the '708patent combines a thermal mass flow controller with a thermal metercoupled to a dome-loaded pressure regulator. Another pilot pressureregulator using an independent gas source loads the dome of the pressureregulator upstream of the thermal mass flow controller. The pilotregulator and the mass flow controller are controlled by amicroprocessor so that inlet pressure is controlled in concert with theflow rate resulting in an inlet pressure independent flow controller.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram depicting an exemplary mass flow meterof the present invention;

[0010]FIG. 2 is an exemplary plot of mass flow through the meter versusfluid pressure illustrating the operation of the present invention;

[0011]FIG. 3 is a somewhat schematic section view depicting an exemplarymechanical system that may be used to implement a mass flow meter asdepicted in FIG. 1;

[0012]FIG. 4 is a block diagram of a meter circuit employed by the massflow meter of FIG. 1;

[0013]FIG. 5 is a detailed block diagram of an exemplary meter circuitthat may be employed by a mass flow meter employing the principles ofthe present invention;

[0014]FIG. 6 is a flow diagram representing one exemplary method ofcalibrating the mass flow meter of FIG. 1;

[0015]FIG. 7 is a plot of mass flow through the meter versus fluidpressure for several different fluid temperatures illustratingcompensation for different fluid temperatures;

[0016]FIG. 8 is an exemplary plot of mass flow through the meter versusfluid pressure illustrating the basic principles of operation of thepresent invention applied to a non-linear mass flow output;

[0017]FIG. 9 is a block diagram of an exemplary flow control systememploying a mass flow meter system of the present invention; and

[0018]FIG. 10 is a block diagram of an alternate embodiment of a flowcontrol system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The following discussion is organized in a number of sections. Inthe first section, the basic operation and theory of the presentinvention will be described in the context of a mass flow meter system.The second and third sections will describe exemplary mechanical andelectrical systems that may be used to implement the present invention.The fourth section will describe one method of calibrating a mass flowmeter system constructed in accordance with the principles of thepresent invention. The fifth section describes the mass flow meterdescribed in the first through fourth section used as part of a massflow controller. The sixth section describes an alternate embodiment ofa mass flow control system. The final section describes additionalconsiderations that are typically taken into account when designing andconstructing a particular implementation of the present invention.

1. Mass Flow Meter System

[0020] Referring initially to FIG. 1 of the drawing, depicted at 20therein is an exemplary mass flow meter system constructed in accordancewith, and embodying, the principles of the present invention. The metersystem 20 comprises a mechanical system 22 and an electrical system 24.The mechanical system 22 comprises a flow restrictor 30 defining arestriction chamber 32 and a pressure balancing system 34. Theelectrical system 24 comprises a pressure sensor 40, a temperaturesensor 42, and a meter circuit 44.

[0021] The mechanical system 22 defines a fluid inlet 50 and a fluidoutlet 52. The inlet 50 and outlet 52 are connected to a source orsupply 54 of pressurized fluid and a destination 56 of that fluid,respectively.

[0022] From the discussion above, it should be apparent that particularsof the source 54 and destination 56 may vary significantly dependingupon the environment in which the meter system 20 is used. For example,in a medical environment, the source 54 may be a bottle of pressurizedgas and the destination 56 may be a mixer that mixes the gas with airand delivered the mixture to a patient using conventional means. In amanufacturing environment, the source 54 may be a converter thatgenerates a supply of gas from raw materials and the destination 56 maybe a reaction chamber in which the gas is used as part of an industrialprocess. In many cases, the supply pressure at the source 54 and backpressure at the destination 56 may be unknown and/or variable.

[0023] The meter system 20 of the present invention is thus intended tobe used as part of a larger system in which pressurized fluid thus flowsfrom the source 54 to the destination 56 through the mechanical system22. The pressure balancing system 34 maintains a constant differentialpressure across the restriction chamber 32.

[0024] As depicted in FIG. 1, the exemplary flow restrictor 30 isvariable. In particular, when the meter system 20 is calibrated the flowrestrictor 30 defines a predetermined geometry and an effectivecross-sectional area of the restriction chamber 32. In the exemplarysystem 20, the flow restrictor 30 may be changed to alter the geometry,and in particular the effective cross-sectional area, of the restrictionchamber 32. In other embodiments of the present invention, the flowrestrictor 30 need not be variable, but instead can be fabricated with apreset geometry and effective cross-sectional area. This may or may notinclude a standard orifice, sonic orifice, laminar flow element ofvarious geometries, or a variable area restriction. The use of a presetor variable flow restrictor may affect the process of calibrating themeter system 20 as will be discussed below.

[0025] The pressure balancing system 34 is preferably a flow controllerthat utilizes a mechanical regulation system to maintain a constantdifferential pressure across the restriction chamber 32 even if thesource and destination pressures are unknown or variable. Suchmechanical flow controllers are disclosed, for example, in U.S. Pat. No.6,026,849 issued Dec. 2, 1999, and copending U.S. patent applicationSer. No. 09/805,708 filed Mar. 13, 2001 and commonly assigned with thepresent application. However, the pressure balancing system 34 may alsobe an electro-mechanical flow controller as disclosed in the '708application. The teachings of the '849 patent and the '708 applicationare incorporated herein by reference.

[0026] The pressure and temperature sensors 40 and 42 are preferablyelectro-mechanical transducers that convert pressure and temperaturevalues into an electrical signal. These sensors 40 and 42 areoperatively connected to the mechanical system 22 to generate electricalsignals indicative of the pressure and temperature, respectively, of thefluid flowing through the mechanical system 22.

[0027] The meter circuit 44 stores or otherwise has access tocalibration data relating mass flow rate to pressure and temperature fora given fluid. The calibration data includes a calibration factorcalculated for a given restrictor 30 and a gas constant determined bythe characteristics of the gas flowing through the meter system 20. Thegas constant is based on the specific gas density or viscosity asrelated to temperature changes.

[0028] Based on the calibration data and the pressure and temperaturesignals, the meter circuit 44 generates a flow output signalcorresponding to the mass flow of fluid through the mechanical system22. The flow output signal may be recorded or displayed or used as partof a larger circuit for controlling fluid flow from the source 54 to thedestination 56.

[0029] Referring now to FIG. 2, depicted therein at 60 is a plot ofpressure versus mass flow through the restrictor 30 when the pressurebalancing system 34 is connected across the restrictor 30 as describedabove. As seen in the figure, the mass flow output increases linearlywith outlet pressure. This curve 60 is an effect of the ideal gas law,which relates volume, mass, temperature, and non-linear compressibilityeffects together as described by the following equation (1):

PV=mRTZ   (1)

[0030] Where:

[0031] P=pressure,

[0032] m=mass,

[0033] V=volume,

[0034] R=Gas Constant (Universal),

[0035] T=temperature, and

[0036] Z=gas compressibility

[0037] (in the following discussion, a “·” above any of these symbolsdenotes a mass of volume flow rate)

[0038] Dividing both sides of the ideal gas law equation by time yieldsthe following rate equation (2):

P{dot over (V)}={dot over (m)}RTZ   (2)

[0039] Solving for the rate equation (2) for mass flow rate yields thefollowing mass flow rate equation (3): $\begin{matrix}{\overset{.}{m} = {\frac{\overset{.}{V}}{RTZ}P}} & (3)\end{matrix}$

[0040] Rearranging the terms of the mass flow rate equation yields thefollowing slope equation (4):$\frac{\overset{.}{\Delta \quad m}}{\Delta \quad P} = {\frac{\overset{.}{V}}{RTZ} = {{{slope}\quad {of}\quad {the}\quad {linear}\quad {portion}\quad {of}\quad {the}\quad {plot}} = {CONSTANT}}}$

[0041] The slope of equation (4) illustrates the relationship betweenthe mass flow rate and pressure for a given system and gas. If thepressure increases, the amount of mass within a certain volume (i.e.,density) will increase proportionally if temperature remains constant.Experimental data showed that the temperature only varied by a fractionof a degree throughout the entire experiment. Since the slope of theplot remained constant, the end result was the volumetric flow rate forthis device remained constant through the entire pressure range untilthe pressure differential pressure (i.e., inlet pressure minus outletpressure) approached a critical value.

[0042] In contrast, traditional flow meter devices that rely on pressuremeasurements must take into account three factors: inlet pressure, inlettemperature, and pressure differential across an orifice. The flow rateacross an orifice, or similar flow restriction, is expressed in generalterms by the following flow rate equation (5): $\begin{matrix}{{\overset{.}{m} = {{K\lbrack {d^{2}\frac{1}{{\sqrt{1 - ( \frac{d}{D} )}}^{4}}} \rbrack}\frac{\sqrt{{Gp}_{1}}}{{ZT}_{1}}\sqrt{p_{1} - p_{2}}}}{{Where}:\begin{matrix}{p_{1} = {{gas}\quad {pressure}\quad {upstream}\quad {of}\quad {the}\quad {restriction}}} \\{p_{2} = {{gas}{\quad \quad}{pressure}\quad {downstream}\quad {of}\quad {restriction}}} \\{T_{1} = {{temperature}{\quad \quad}{upstream}\quad {of}\quad {restriction}}} \\{D = {{flow}\quad {passage}\quad {diameter}}} \\{d = {{restriction}\quad {hydraulic}\quad {diameter}\quad ( {{effective}\quad {flow}\quad {diameter}} )}} \\{G = {{specific}\quad {gravity}\quad {or}\quad {normalized}\quad {molecular}\quad {weight}\quad {of}\quad {gas}}} \\{Z = {{compressibility}\quad {factor}\quad {of}\quad {gas}}}\end{matrix}}} & (5)\end{matrix}$

[0043] The term K in this flow rate equation (5) is a factor that isdetermined experimentally during the calibration of a given restriction.The term K is dependent on the geometry of the restriction and expansionfactors of the gas such as Joule-Thompson cooling/heating (i.e., thechange in temperature caused by a sudden change in pressure). The flowrate equation (5) is only valid for low flows or restrictions that donot create large gas velocities inside of them. When the speed of thegas approaches the speed of sound, the bulk speed of the gas moleculesis larger than the speed at which pressure can travel through themedium. The flow properties take on significantly differentrelationships and are called compressible flows, sonic flows, or chokedflows.

[0044] Therefore, traditional flow controllers relying on pressure dropsemploy two pressure sensors and a temperature sensor. Such traditionalflow controllers must also have relatively sophisticated electronicscapable of calculating flow rate by measuring both pressures,calculating the pressure difference (with custom op amps (analog) or bymeans of a programmed digital microprocessor and the needed analog todigital converters), and most importantly, by calibrating the device tofind out the term K.

[0045] With the approach of the present invention, the equation to solveto obtain flow would look like one the following equations (6) or (7):$\begin{matrix}{{\overset{.}{m} = {\frac{K}{R}\frac{P_{1}}{T_{1}}\quad ( {{for}\quad a\quad {laminar}\quad {flow}\quad {restriction}} )}}{or}} & (6) \\{\overset{.}{m} = {\frac{K}{R}\frac{\sqrt{P_{1}}}{T_{1}}\quad ( {{for}{\quad \quad}{an}\quad {orifice}\quad {type}\quad {restriction}} )}} & (7)\end{matrix}$

[0046] Where: K is the calibration factor and is determined duringcalibration as will be discussed below. It should be noted that theconstant, R, in Equations (6-10) is not the Universal constant, R, ofEquations (1-4). Rather, it is a gas-dependent constant that varies forlaminar flow or orifice type restrictions.

[0047] The case of some gases where non-ideal compressibility must betaken into account, the following equations (8) and (9) may be used:$\begin{matrix}{{\overset{.}{m} = {\frac{{KP}_{1}}{{RT}_{1}}( \frac{1}{Z( {P,T} )} )\quad ( {{for}\quad a\quad {laminar}\quad {flow}\quad {restriction}} )}}{or}} & (8) \\{\overset{.}{m} = {\frac{K\sqrt{P_{1}}}{{RT}_{1}}( \frac{1}{Z( {P,T} )} )\quad ( {{for}{\quad \quad}{an}\quad {orifice}\quad {type}\quad {restriction}} )}} & (9)\end{matrix}$

[0048] Where: Z(P,T) is the compressibility factor that is dependent onpressure and temperature.

[0049] As can be seen by a comparison of equation (5) with any of theequations (6), (7), (8), or (9), the present invention greatlysimplifies the relationship of mass flow rate to pressure andtemperature. In most cases, compressibility does not need to be obtaineddirectly, because the controller will be calibrated such thatcompressibility is accounted for in the calibration sequence.

[0050] At a given temperature and pressure, the gas may already beshowing some non-ideal compressibility that will be inherent in themeasurement taken by the flow standard during calibration. In addition,the term R is gas specific, so only the gas specific constant needs tobe entered before or during calibration to have a highly accurate massflow measurement. The calibration sequence may be implemented as will bedescribed below with reference to FIG. 5.

[0051] After the calibration factor K is calculated using thecalibration sequence, mass flow may be measured using only the followinglinear slope equation (10) defining the slope of the plot 60 depicted inFIG. 2:

Y=mx+b   (10)

[0052] Where: y=mass flow, x=measured pressure, m=K/RT, and b is thezero offset.

[0053] With the foregoing basic understanding of the meter system 20 inmind, the various components of this system will now be described infurther detail below.

II. Mechanical System

[0054] Referring now to FIG. 3 of the drawing, depicted in detailtherein is the mechanical system 22 of the exemplary flow meter system20. The restrictor 30 of the mechanical system 22 is formed by a mainbody assembly 120. The main body assembly 120 defines a main passageway130 having an inlet 132 and an outlet 134 and defining the restrictionchamber 32. The restriction chamber 32 is arranged between the inlet 132and the outlet 134.

[0055] As generally discussed above, the mass flow meter system 20measures the mass flow of fluid that flows through the main passageway130 from the inlet 132 to the outlet 134 using pressure and temperaturesignals generated by the pressure sensor 40 and the temperature sensor42. The fluid flowing through the meter system 20 will be referred toherein as the metered fluid. Seals are formed at the junctures of thevarious parts forming the mechanical system 22 such that metered fluidflows only along the paths described herein; these seals are or may beconventional and thus will not be described in detail.

[0056] The exemplary main body assembly 120 comprises a main body member140 and, optionally, a variable orifice assembly 142. The main bodymember 140 defines at least a portion of the main passageway 130, theinlet 132, and the outlet 134. The main body member comprises an inletsection 144, an outlet section 146, and a intermediate section 148.

[0057] The main body member 140 further defines first and secondbalancing ports 150 and 152 located upstream and downstream,respectively, of the variable orifice assembly 142. The first and secondbalancing ports 150 and 152 allow fluid communication between thepressure balancing system 24 and the inlet and outlet sections 144 and146, respectively, of the main passageway 130. The first balancing port150 and second balancing port 152 are connected to input and outputports 154 and 156, respectively, of the pressure balancing system 34.

[0058] The exemplary pressure and temperature sensors 40 and 42 used bythe meter system 20 are arranged to detect the pressure and temperatureof the metered fluid flowing through the main passageway 130. Inparticular, the main body member 140 defines first and second test ports160 and 162 arranged in the outlet section 146 of the main body member140. The test ports 160 and 162 may, however, be arranged in the inletand/or intermediate sections 144 or 148 of the body member 140 inanother embodiment of the present invention.

[0059] The sensors 40 and 42 are or may be conventional and are insertedor threaded into the test ports 160 and 162. Seals are conventionallyformed between the sensors 40 and 42 and the test ports 160 and 162. Soattached to the main body member 140, the sensors 40 and 42 generateelectrical pressure and temperature signals that correspond to thepressure and temperature of the metered fluid immediately adjacent tothe test ports 160 and 162.

[0060] The inlet, outlet, and restriction sections 144, 146, and 148 ofthe main body member 140 serve different functions and thus havedifferent geometries. The inlet and outlet sections 144 and 146 arethreaded or otherwise adapted to allow a fluid-tight connection to bemade between the main body member 140 and the source 54 and destination56 of the metered fluid. The effective cross-sectional areas of inletand outlet sections 144 and 146 are not crucial to any implementation ofthe present invention except that the flow of metered fluid to the fluiddestination 56 must meet predetermined system requirements. In theexemplary main body assembly 120, the inlet and outlet sections 144 and146 define cylindrical inlet and outlet internal wall surfaces 170 and172 and have substantially the same diameter and effectivecross-sectional area.

[0061] The intermediate section 148 of the main body member 140 servesto restrict the flow of metered fluid through the main passageway 130while still allowing the flow of metered fluid to meet the systemrequirements. The effective cross-sectional area of at least a portionof the intermediate section 148 of the main passageway 130 is thussmaller than that of the inlet and outlet sections 144 and 146. Inparticular, the intermediate section 148 is defined at least in part byan internal restriction wall 180 of the main body member 140. Therestriction wall 180 is substantially cylindrical and has a diametersmaller than that of the inlet and outlet wall surfaces 170 and 172.

[0062] The meter system 20 of the present invention may be manufacturedwithout the optional variable orifice assembly 142. In this case, therestriction wall 180 of the main body member 140 defines the restrictionchamber 32. The main body member 40 must be manufactured to tighttolerances and/or the calibration data may need to be calculated foreach main body member 140 to account for variations in the restrictionportions defined by individual main body members if a variable orificeassembly is not used.

[0063] If a variable orifice assembly 142 is used, the restrictionchamber 32 associated with a given main body member 140 may be alteredto calibrate the given main body member 140. Any number of mechanismsmay be used to alter the geometry of the restriction chamber 32.

[0064] In the meter system 20, the exemplary variable orifice assembly142 comprises a tube member 220 having an internal surface 222. Theinternal surface 222 of the tube member 220 defines the effectivecross-sectional area of the restriction chamber 32.

[0065] In some situations, the tube member 220 may be made of a rigidmaterial such as some metals or polymers. In this case, the tube member220 is made in a plurality of predetermined configurations eachcorresponding to a restriction chamber 32 having a differentpredetermined cross-sectional area. One of these predeterminedconfigurations is selected to obtain a desired geometry of therestriction chamber 32.

[0066] The exemplary tube member 220 is, however, made of a deformablematerial such that, when the tube member 220 is deformed, the effectivecross-sectional area of the restriction chamber 32 is changed. Theexemplary tube member 220 is made of metal, but polymers, naturalrubber, or other materials may be used depending upon the circumstances.In this respect, the tube member 220 may be made of elastic (e.g.,polymers or natural rubber) or non-elastic (e.g., metal) material.

[0067] The variable orifice assembly 142 used by the exemplary metersystem 20 further comprises a compression wedge 224, a compression shim226, first and second chevron members 228 and 230, and a compression nut232 having a threaded surface 234.

[0068] To accommodate this variable orifice assembly 142, theintermediate section 148 of the exemplary main body member 140 comprisesthe following interior walls in addition to the restriction wall 180: atube seat wall 240, a compression wall 242, a spacing wall 244, and athreaded wall 246. The tube seat wall 240 is located upstream of therestriction wall 180 described above and is generally cylindrical. Thecompression wall 242 is located upstream of the tube seat wall and isgenerally conical. The spacing wall 244 is located upstream of thecompression wall and is generally cylindrical. The threaded wall 246 islocated upstream of the spacing wall and is threaded to mate with thethreaded surface 232 of the compression nut 230.

[0069] Axial rotation of the compression nut 230 relative to the bodymember 140 thus causes the nut 230 to be displaced along a longitudinalaxis A of the body member 140 towards the restriction wall 180. As thenut 230 moves towards the restriction wall 180, the nut 230 applies aforce on the compression wedge 224 through the chevron members 228 and230 and compression shim 226. The compression wedge 224 comprises aconical outer surface 250. The outer surface 250 of the compressionwedge 224 engages the compression wall 242 such that the wedge 224 movesradially inwardly towards the longitudinal axis A. The inward movementof the compression wedge 224 deforms, as generally described above, thetube member 220 to alter the effective cross-sectional area of therestriction chamber 32.

III. Electrical System

[0070] Referring now to FIG. 4 of the drawing, depicted in detailtherein is one exemplary embodiment of a meter circuit 44 used as partof the electrical system 24 of the exemplary flow meter system 20. Themeter circuit 44 comprises first, second, and third summing and scalingsystems 320, 322, and 324. The first summing and scaling system 320combines the calibration factor and raw pressure signal to obtain acalibrated pressure signal. The second summing and scaling system 322combines the raw temperature signal and the gas constant input to obtaina compensated temperature signal. The third summing and scaling system324 combines the calibrated pressure signal and the compensatedtemperature signal to obtain the flow output signal.

[0071] The design details of the summing and scaling systems 320-324will be determined by the specific environment in which the meter system20 is to be used. Typically, these systems 320-324 will comprise signalspecific components and a summing and scaling amplifier. The signalspecific components convert a raw input signal in either analog ordigital form into a digital or analog conditioned signal suitable foruse by the summing and scaling amplifier associated with the signalspecific components. The summing and scaling amplifier in turn isdesigned to generate a scaled signal based on the conditioned inputsignals.

[0072] The meter circuit 44 may be implemented using discrete circuitcomponents, an application specific integrated circuit (ASIC), softwarerunning on an integrated processor such as a general purposemicrocomputer or a digital signal processor, or a combination of thesemethods. The exact nature of any given implementation the electricalsystem 24 will depend upon such factors as manufacturing costs, thedesigners background and experience, and the operating environment ofthe meter system 20. For example, in an embodiment of the presentinvention implemented with a digital signal processor (“DSP”), the DSPpreferably comprises a memory unit having look-up tables that storecalibration conditions including but not limited to the originalcalibration conditions for the meter. This data is useful for referenceback to original conditions in the case of pressure and/or temperaturesensor drift. The data is also useful for conducting diagnosticprocedures to determine whether the meter requires calibration or otherservice. Additionally, the DSP memory unit preferably has a look-uptable of fluid viscosity vs. temperature for one or more fluids. Thisdata is useful for use in compensating for changes in fluid temperature.

[0073] Referring now to FIG. 5, depicted therein is one exemplary metercircuit 44 adapted to generate the flow output signal based on analoginput signals. As shown in FIG. 5, the first summing and scaling system320 comprises a signal conditioning module 330, an optional arithmeticlogic unit 332, an optional linearization amplifier 334, and a first andsecond summing and scaling amplifiers 336 and 338.

[0074] The raw pressure signal is initially filtered and amplified bythe signal conditioning module 330. If necessary, the filtered pressuresignal is then applied to one or both of the arithmetic logic unit 332and linearization amplifier 334 and then to the first summing andscaling amplifier 336. If the arithmetic logic unit 332 andlinearization amplifier 334 are not required, the filtered pressuresignal is directly passed to the first summing and scaling amplifier336. The second summing and scaling amplifier 338 generates acalibration signal based on the calibration factor. The pressure signaland calibration signal are then applied to the first summing and scalingamplifier 336 to obtain the processed pressure signal.

[0075] The second summing and scaling system 322 comprises a signalconditioning module 340, a scaling and gain amplifier 342, and a summingand scaling amplifier 344. The signal conditioning module 340 filtersand amplifies the temperature signal to obtain a filtered temperaturesignal. The scaling and gain amplifier 342 generates a gas constantsignal based on the gas constant input. The summing and scalingamplifier 344 generates the processed temperature signal based on thefiltered temperature signal and the gas constant signal.

[0076] The third summing a scaling system 324 comprises a summing andscaling amplifier 350 and a buffer amplifier 352. The summing andscaling amplifier generates a flow signal based on the calibratedpressure signal and the compensated temperature signal as generallydescribed above. The buffer amplifier 352 generates the flow outputsignal based on the flow signal.

IV. Calibration Process

[0077] Referring now to FIG. 5 of the drawing, depicted therein at 360is a flow diagram of one exemplary process for calibrating the metersystem 20 described above. In the following discussion, the particularmeter system 20 being calibrated will be referred to as the DUT.

[0078] The first step 362 of the calibration process is to connect theflow restrictor 30 of the DUT in series with a calibrated meter system.A negative gauge pressure or vacuum is then applied at step 364 to theoutlet of the flow restrictor 30 of the DUT, and the electronics of themeter system 20 are set to zero.

[0079] The next step 366 is to apply pressure upstream of the DUT tocreate flow through the DUT. The flow is measured using the calibratedmeter system. The gas specific gas constant input is then applied atstep 368 to the electronic portion 24 using conventional means such as adigital serial input and/or a set of one or more switches that can beconfigured to generate the appropriate gas constant input.

[0080] The maximum flow range is then obtained at step 370 by selectingan appropriate geometry of the restriction cavity 32 by any one of themethods described above.

[0081] The flow is then decreased at step 372 to ten percent of themaximum flow setting of the DUT. The pressure and temperature signalsassociated with that flow are then read and stored. At step 374, theflow rate is increased in increments of ten percent up to one hundredpercent. The pressure and temperature signals associated with eachincremental increase in flow rate are measured and stored. The slope ofplot of the pressure signal versus the mass flow rate measured at step378 by the calibrated meter system is measured and stored as thecalibration factor using conventional means such as a trim pot ordigital serial input.

[0082] Referring to FIG. 6, depicted at 380 a, 380 b, and 380 c thereinare exemplary plots of pressure signal versus mass flow rate for severaltemperatures. The meter circuit 44 generates the flow signal outputsignal based on the pressure/mass flow plots created by the calibrationfactor and gas constant input.

[0083] Referring now to FIG. 7, depicted therein is a plot 382 of thepressure signal versus mass flow rate in which the relationship betweenthe pressure signal and mass flow rate is non-linear. For example, thisrelationship may be non-linear in the case of an orifice.

[0084] If the pressure/mass flow rate relationship is non-linear, thefiltered pressure signal will be passed through one or both of thearithmetic logic unit 332 and linearization amplifier 334. Thearithmetic logic unit 332 and linearization amplifier 334 implement afunction that compensates for the non-linearity of the pressure/massflow rate relationship. For example, the signal conditioning circuitrymay perform one or both of a “piecewise linearization” function or asquare root function on the filtered pressure signal to obtain thecompensated pressure signal. In particular, referring back to FIG. 8,depicted at 384 is a curve corresponding to the inverse of thenon-linear curve 382. A curve 386 represents the midpoint of the curves382 and 384 and can be used in the linear slope equation (10) describedabove.

[0085] In practice, the meter circuit 44 is preferably manufactured withboth the arithmetic logic unit 332 and linearization amplifier 334 and,as shown in FIG. 5, switches 390 and 392 configured to allow either ofthese circuit elements 332 and 334 to be removed from the circuit 44.The use of the switches 390 and 392 thus allows the production of astandard meter circuit 44 that can easily be customized for a particularenvironment.

V. Mass Flow Control System

[0086] As generally described above, the mass flow meter of the presentinvention described above has numerous applications. It can be usedalone simply to measure mass flow rate of a wide variety of fluids at awide variety of flow rates. It can be used as part of a larger system ofprocessing or administering fluids where accurate mass flow rates areimportant. It can also be combined with other components to obtain amore complex stand alone device.

[0087] Described in this section with reference to FIG. 9 is anexemplary mass flow control system 420 that incorporates the exemplarymass flow meter 20 described above. The mass flow control system 420 isa stand alone device that not only measures mass flow rate but allowsthis flow rate to be controlled with a high degree of accuracy for awide variety of fluids and flow rates.

[0088] The mass flow control system 420 incorporates the flow metersystem 20 described above, and the meter portion of the flow controlsystem 420 will not be described again except to the extent necessaryfor a complete understanding of the flow control system 420.

[0089] In addition to the flow meter system 20, the flow control system420 comprises a valve control feedback loop system 422 and a flowcontroller system 424. The flow controller system 424 is arranged inseries with the flow meter system 20 such that the flow controllersystem 424 determines the mass flow of fluid through the flow metersystem 20.

[0090] Preferably, the flow controller system 420 is a mechanical orelectro-mechanical flow controller such as is described in the '849patent and '708 application cited above. The flow controller system 424may, however, be any flow controller system that can increase ordecrease the flow of fluid through the system 420 under electrical ormechanical control.

[0091] In the present invention, the flow signal generated by thesumming and scaling amplifier 350 of the third summing and scalingsystem 324 is applied to the valve control feedback loop system 422. Thevalve control feedback loop system 422 compares the flow signal with adesired flow rate signal. The desired flow rate signal may be preset ormay be changed as required by the circumstances. For example, in amedical setting, a doctor may prescribe that a gas be applied to apatient at a predetermined flow rate. The predetermined flow ratedetermined by the doctor would be converted into the desired flow ratesignal.

[0092] Based on the difference between the desired flow rate signal andthe flow signal generated by the flow meter system 20, the valve controlfeedback loop system 422 generates a flow control signal that controlsthe flow controller system 424. If the flow controller system 424 is amechanical system, the flow control signal will be in the form ofmechanical movement (rotational, translational) that operates the flowcontroller signal to increase or decrease the fluid flow rate throughthe system 424. If the system 424 is an electro-mechanical system, theflow control signal may take the form of an electrical signal that isconverted to mechanical movement at the system 424.

[0093] The combination of the flow controller system 424 and the flowmeter system 20 results in the fluid output of the system 420 beingcontrollable to a high degree of accuracy.

VI. Alternative Embodiment of Mass Flow Control System

[0094] An alternative embodiment of the mass flow control systemdescribed above and in FIG. 9 is shown in FIG. 10. Specifically, theflow controller 524 comprises either a piezoelectric actuator control ora solenoid actuator control 526 coupled to a valve 528. The actuatorcontrol 526 delivers a signal 530 to the meter circuit 44. If theactuator control 526 is a solenoid actuator control then the signal 530is a current signal is converted to a voltage signal. If the actuatorcontrol 526 is a piezoelectric actuator control, the signal 530 is avoltage signal from an integrated strain gauge. In either instance, thesignal 530 can be identified as V_(pm), i.e., Voltage (prime mover).

[0095] The signal 530 represents the relationship between the Lorentzforce generated in the actuator control 526 by changes in pressure inthe valve 528. Hence, the signal 530 can be used as an indirect pressureindicator replacing, augmenting and/or calibrating the pressuretransducer 40 in FIG. 1. For example, in FIG. 10, the pressuretransducer 40 is not present and the signal 530 is used in its stead.The signal 530 can also be used as a diagnostic indicator to verify thatthe value of R for a given flow restriction has not changed.

[0096] Preferably, the meter circuit 44 for the mass flow control systemshown in FIG. 10 has at least 128 kilobytes of memory using EEPROM. Thememory for the meter circuit 44 should contain a look up table of valuesof V_(pm) for incremental mass flow rates for various gases and/or flowrestrictions. This look-up table preferably represents values for theequation: $\begin{matrix}{\overset{.}{m} = \frac{{KV}_{pm}}{RZ}} & (11)\end{matrix}$

[0097] Thus, an alternative embodiment using a signal 530, V_(pm), tomeasure changes in pressure in the system and to control the valve 528in the mass flow control system is described above.

VII. Additional Considerations

[0098] A designer will typically design a particular implementation ofthe present invention by initially determining the operating environmentin which the flow meter system is to be used. The operating environmentwill include the properties of the fluid itself, the expected range offluid input and output pressures, the ambient conditions, the tolerancefor error, and the like. The designer may also consider commercialfactors such as cost.

[0099] The properties of many of both the mechanical and electricalcomponents of the present invention will be changed depending upon thecircumstances to “tune” a specific flow meter system for a particularuse.

[0100] For example, the restriction chamber and inlet and outletopenings may be selected based on the type of fluid, expected inletpressures, and desired flow rates.

[0101] In addition, the materials used for the various components mustbe selected based on the pressures and types of fluids expected. Forexample, for air at low pressures, plastic may be used for many of thecomponents. For caustic fluids and higher pressures, steel or stainlesssteel may be used.

[0102] The electronics will also be customized for a particularenvironment. For example, the implementation details of the varioussumming and scaling systems described above will be determined once theparticular operating environment is defined.

[0103] Accordingly, the present invention may be embodied in forms otherthan those described herein without departing from the spirit oressential characteristics of the invention. The present embodiments aretherefore to be considered in all respects as illustrative and notrestrictive, the scope the invention being indicated by the appendedclaims rather than by the foregoing description; and all changes whichcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

What is claimed is:
 1. A mass flow meter comprising: an inlet with adiameter and an outlet with a diameter; a flow restrictor having arestriction chamber and a pressure balancing system interposed betweenthe inlet and outlet; the restriction chamber having a cylindricalrestriction wall with a diameter less than the diameters of the inletand outlet; and, a pressure sensor and a temperature sensor upstreamfrom the flow restrictor providing input to a meter circuit havingcalibration data; whereby the meter circuit generates a flow outputsignal based on the calibration data and the input from the pressuresensor and temperature sensor.
 2. The mass flow meter of claim 1 wherethe restriction wall defines the restriction chamber.
 3. The mass flowmeter of claim 1 where the restriction chamber further comprises: aconical compression wall located upstream of the restriction wall; avariable orifice assembly, the variable orifice assembly having adeformable tube member having an internal surface defining an effectivecross-sectional area of the restriction chamber, a compression wedge, acompression shim, first and second chevron members, and a compressionnut; whereby axial rotation of the compression nut displaces the nuttowards the restriction wall, as the nut moves toward the restrictionwall; the nut applies a force on the compression wedge through the firstand second chevron members and compression shim and forces thecompression wedge radially inwards deforming the deformable tub memberand reducing the effective cross-sectional area of the restrictionchamber.
 4. The mass flow meter of claim 1 where the meter circuitfurther comprises: a first summing and scaling system that combines thecalibration data and the pressure signal to obtain a calibrated pressuresignal; a second summing and scaling system that combines thetemperature signal and a gas constant to obtain a compensatedtemperature signal; and, a third summing and scaling system thatcombines the calibrated pressure signal and the compensated temperaturesignal to obtain the flow output signal.
 5. The mass flow meter of claim4 where the meter circuit is implemented on an application specificintegrated circuit.
 6. The mass flow meter of claim 4 where the metercircuit is implemented on a digital signal processor.
 7. The mass flowmeter of claim 6 where the meter circuit further comprises a memory unitfor storing calibration conditions.
 8. The mass flow meter of claim 4where the first summing and scaling system comprises: a signalconditioning module; an arithmetic logic unit; an optional linearizationamplifier; and, a first and second summing and scaling amplifier.
 9. Themass flow meter of claim 4 where the second summing and scaling systemcomprises: a signal conditioning module; a scaling and gain amplifier;and, a summing and scaling multiplier.
 10. The mass flow meter of claim4 where the third summing and scaling systems comprises: a summing andscaling amplifier; and, a buffer amplifier that generates the flowoutput signal.
 11. The mass flow meter of claim 8 where the metercircuit further comprises a switch for removing either the arithmeticlogic unit or the linearization amplifier from the circuit.
 12. A massflow control system comprising: a flow controller capable of controllinga flow rate through the mass flow control system; said controllerconnected in series with a mass flow meter; said mass flow metercomprising an inlet with a diameter and an outlet with a diameter; aflow restrictor having a restriction chamber and a pressure balancingsystem interposed between the inlet and outlet; the restriction chamberhaving a cylindrical restriction wall with a diameter less than thediameters of the inlet and outlet; and, a pressure sensor and atemperature sensor upstream from the flow restrictor providing input toa meter circuit having calibration data; whereby the meter circuitgenerates a flow output signal based on the calibration data and theinput from the pressure sensor and temperature sensor; said mass flowmeter is connected to a valve control feedback loop system; whereby themass flow meter sends the flow output signal to the valve controlfeedback loop system and said feedback system compares the flow outputsignal with a predetermined flow rate to generate a flow control signal;said valve control feedback loop system is connected to the flowcontroller; whereby said feedback system sends the flow control signalto the flow controller to operate the flow controller to change the flowrate.
 13. The mass flow control system of claim 12 where the restrictionwall defines the restriction chamber.
 14. The mass flow control systemof claim 12 where the restriction chamber further comprises: a conicalcompression wall located upstream of the restriction wall; a variableorifice assembly, the variable orifice assembly having a deformable tubemember having an internal surface defining an effective cross-sectionalarea of the restriction chamber, a compression wedge, a compressionshim, first and second chevron members, and a compression nut; wherebyaxial rotation of the compression nut displaces the nut towards therestriction wall, as the nut moves toward the restriction wall; the nutapplies a force on the compression wedge through the first and secondchevron members and compression shim and forces the compression wedgeradially inwards deforming the deformable tub member and reducing theeffective cross-sectional area of the restriction chamber.
 15. The massflow control system of claim 12 where the meter circuit furthercomprises: a first summing and scaling system that combines thecalibration data and the pressure signal to obtain a calibrated pressuresignal; a second summing and scaling system that combines thetemperature signal and a gas constant to obtain a compensatedtemperature signal; and, a third summing and scaling system thatcombines the calibrated pressure signal and the compensated temperaturesignal to obtain the flow output signal.
 16. The mass flow controlsystem of claim 12 where the meter circuit is implemented on anapplication specific integrated circuit.
 17. The mass flow meter ofclaim 15 where the meter circuit is implemented on a digital signalprocessor.
 18. The mass flow meter of claim 15 where the first summingand scaling system comprises: a signal conditioning module; anarithmetic logic unit; an optional linearization amplifier; and, a firstand second summing and scaling amplifier.
 19. The mass flow meter ofclaim 15 where the second summing and scaling system comprises: a signalconditioning module; a scaling and gain amplifier; and, a summing andscaling multiplier.
 20. The mass flow meter of claim 15 where the thirdsumming and scaling systems comprises: a summing and scaling amplifier;and, a buffer amplifier that generates the flow output signal.
 21. Amethod for calibrating a flow meter; said flow meter comprising an inletwith a diameter and an outlet with a diameter; a flow restrictor havinga restriction chamber and a pressure balancing system interposed betweenthe inlet and outlet; the restriction chamber having a variable orificeassembly; a cylindrical restriction wall with a diameter less than thediameters of the inlet and outlet; and, a pressure sensor and atemperature sensor upstream from the flow restrictor providing input toa meter circuit having calibration data; whereby the meter circuitgenerates a flow output signal based on the calibration data and theinput from the pressure sensor and temperature sensor; the steps of saidmethod comprising: connecting the flow meter to a referenced flowstandard; applying a negative gauge pressure to the outlet; setting azero point on the meter circuit; applying pressure upstream of themeter; measuring a flow; entering a gas specific constant into thecalibration data of the meter circuit; obtaining a maximum flow range byadjusting the variable orifice assembly for a given inlet pressure;decreasing the flow to ten percent of maximum flow; measuring thepressure and temperature and storing results in the meter circuit;increasing the flow by ten percent increments up to the maximum flow;measuring the pressure and temperature at each ten percent increment andstoring the results in the meter circuit; calculating a slope of a graphof pressure versus mass flow rate from the results stored in the metercircuit; setting the meter circuit by using the slope.
 22. The method ofclaim 21 where the meter circuit further comprises an arithmetic logicunit and a linearization amplifier and, where the graph is non-linear,the steps of said method further comprise: performing a piecewiselinearization function on a filtered pressure signal to obtain acompensated pressure signal for use in setting the meter circuit. 23.The method of claim 21 where the method further comprises the followingstep: measuring a current and storing results in the meter circuit as apressure.
 24. A mass flow meter comprising: an inlet and an outlet; aflow restrictor and a pressure balancing system interposed between theinlet and outlet, whereby the system maintains a constant volumetricflow through the flow restrictor; a pressure sensor and a temperaturesensor upstream from the flow restrictor providing input to a metercircuit, P and T respectively, having a stored calibration factor K, astored gas constant R and zero offset; whereby the meter circuitgenerates a flow output signal based on the stored calibration factor,the stored gas constant, the zero offset and the input from the pressuresensor and temperature sensor according to the equation: y=mx+b where yis mass flow, x is P, m is KR/T, and b is the zero offset.
 25. A massflow control system comprising: a flow controller capable of controllinga flow rate through the mass flow control system; said controllerconnected in series with a mass flow meter and generating a voltagesignal to a meter circuit; said mass flow meter comprising an inlet witha diameter and an outlet with a diameter; a flow restrictor having arestriction chamber and a pressure balancing system interposed betweenthe inlet and outlet; the restriction chamber having a cylindricalrestriction wall with a diameter less than the diameters of the inletand outlet; and, a temperature sensor upstream from the flow restrictorproviding input to the meter circuit; said meter circuit havingcalibration data whereby the meter circuit generates a flow outputsignal based on the calibration data, the input from the temperaturesensor and the voltage signal; said mass flow meter is connected to avalve control feedback loop system; whereby the mass flow meter sendsthe flow output signal to the valve control feedback loop system andsaid feedback system compares the flow output signal with apredetermined flow rate to generate a flow control signal; said valvecontrol feedback loop system is connected to the flow controller;whereby said feedback system sends the flow control signal to the flowcontroller to operate the flow controller to change the flow rate. 26.The mass control system of claim 25 where the flow controller comprisesa piezoelectric actuator control and a valve.
 27. The mass controlsystem of claim 25 where the flow controller comprises a solenoidactuator control and a valve.
 28. The mass control system of claim 25where the flow controller generates a current signal which is convertedto the voltage signal.